1. Field of the Invention
The invention pertains to the field of semiconductor devices. More particularly, the invention pertains to ultrahigh-speed optoelectronic devices.
2. Description of Related Art
High-speed optoelectronic devices are broadly applied in modem datacommunication and telecommunication systems.
These devices can be separated into two categories: those directly modulated by injection of current into the gain region, and those externally modulated. Direct-modulation offers the advantage of low cost but requires very high photon densities in the resonant cavity. For, example, edge-emitting lasers operating at 40 Gb/s have been reported.
The intrinsic speed is defined by the so-called “−3 dB” bandwidth, which is roughly proportional to the relaxation-oscillation frequency:
                                          f            r                    =                                    1                              2                ⁢                π                                      ⁢                                                                                g                    n                                    ⁢                                      p                    0                                                                    τ                  p                                                                    ,                            (        1        )            where gn denotes the differential gain, p0 is the average photon density in the cavity, and τp is the cavity photon lifetime.
A first way to increase the laser bandwidth is to increase the pump current density thereby increasing the photon population of the cavity, for example, by reducing the surface area of the device for the same total current. Under pulsed excitation relaxation, an oscillation frequency as high as 70 GHz has been demonstrated in a pulsed regime at room temperature under applied voltage of 15 volts. Continuous wave (CW) operation at very high current and dissipated power densities was not possible. Commonly accepted limitations for all types of semiconductor laser diodes are at ˜0.2 mW/μm2 of thermal power per surface area for heat dissipation. The limiting optical power is close to ˜1 mW/μm2. Maximum current densities assuming 30% wall-plug efficiency are about ˜2 mA/μm2 or 20 kA/cm2.
A second way to increase the laser bandwidth is to increase the differential gain by lowering the dimensionality of the active layer. Modification of the density of states provides two key advantages. First, differential efficiency and threshold current can be made temperature stable in a wide temperature range.
More importantly, converting from quantum wells (QWs) to quantum dots (QDs) enables edge emitting lasers to reach 10 Gb/s operation at current densities of ˜1 kA/cm2, where conventional InP quantum well devices of this spectral range just start to lase. The bandwidth increases approximately as a square root of current density. At a current density of approximately 20 kA/cm2, a quantum dot laser has the potential to reach 40 Gbps direct modulation if parasitic temperature-related effects and gain saturation effects are avoided. A quantum well device approaches the same frequency at about 3-4 times higher current densities.
Doping the cavity simplifies carrier trapping into the active media and reduces radiative lifetime increasing differential gain.
Shifting the lasing photon energy beyond the bandedge of the semiconductor to the gain continuum is possible for distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs). In VCSELs this approach is particularly attractive, enabling device operation in the range of high gain, while avoiding gain saturation effects within the low-gain tail states.
Another very powerful but less exploited approach addresses optical mode engineering in the VCSEL cavity. Many disadvantages of laser diodes originate from parasitic radiative modes, which cause enhanced radiative leakage, thus preventing quickly approaching a population inversion. The same modes cause effective depletion of the vertical cavity gain below threshold, once the device reaches the population inversion condition. These parasitic processes are most pronounced in the solutions aimed at high-speed operation; in p-doped active media and in cavities with the cavity dip shifted into the gain continuum, where parasitic radiative recombination and gain leakage are particularly strong.
To prevent this effect, a photonic crystal-confined VCSEL may be used. In this case, radiative and gain leakage are partly prohibited by lack of photon states in the photonic crystal surrounding the cavity. Thus parasitic radiative recombination and gain are prohibited. This approach enables practically thresholdless lasers. Once the stimulated emission is channeled in a single mode, much higher gain is possible.
A disadvantage of this approach is its much higher cost and precise processing tolerances, related, for example, to deep ion etching of the distributed Bragg reflector (DBR) layers. The effect of suppressed parasitic leakage may also be achieved by choosing a specific VCSEL design.
None of the approaches for high-speed operation in the prior art address problems of parasitic capacitance, inductance and resistance in VCSELs.
In spite of the fact that low threshold quantum well and quantum dot devices exist, parasitic capacitance, inductance and resistance in VCSELs limit their high-speed performance, even when intrinsic parameters (relaxation oscillations) indicate the possibility for a much faster bit rate.
It is difficult to reduce parasitics in VCSELs and still provide certain aperture sizes, certain thicknesses of the Bragg reflector regions, certain surface areas of the current pads and unavoidable high differential capacitance of the p-n junction under direct bias. Simultaneously approaching low threshold current, high frequency response, low power dissipation and high reliability is difficult for directly-modulated VCSELs.
The problem is usually addressed with careful optimization of the processing technology. The best bandwidth achieved in the prior art so far (17.0 GHz) was realized in a device operating at 4.5 mA with a 7 μm aperture (˜12 kA/cm2). Dissipated power was 8.1 mW.
Faster intrinsic modulation responses of the device (larger bandwidths at the same current density) add more flexibility to achieve necessary compromises to reduce the parasitic effects that hinder time response. Modulation bandwidth of 40 GHz has been demonstrated in GaAs-based edge-emitting lasers and no fundamental limitations for VCSELs exist. High-speed VCSELs are used in high-bit rate long-distance LAN networks based on multimode fibers. However, a principal limitation is that the device must be operated at high current densities to achieve high modulation speed. 40 Gb/s VCSELs have not yet been demonstrated.
In contrast, indirect modulation using elecrooptic effects under reverse bias has long been known in ultrahigh-speed transmitters operating at 40-60 Gb/s. For example, a 40-Gb/s open eye diagram of the electroabsorption modulator after 700-km transmission has been demonstrated.
Once the need for direct modulation is abandoned, ultrahigh-speed signal management becomes much easier. 60-100 GHz pin diode photodetectors using large mesa devices as well as other devices are known in the art.
U.S. Pat. No. 6,285,704, “FIELD MODULATED VERTICAL CAVITY SURFACE-EMITTING LASER WITH INTERNAL OPTICAL PUMPING”, issued Sep. 4, 2001, proposes a photopumped VCSEL. This VCSEL may be modulated by using an external electrical field applied perpendicular to the active layer, employing the Stark-effect to deliberately change the bandgap of the active layer and hence move the emission wavelength into and out of resonance with the optical cavity formed between the top and bottom mirrors. The optical output is therefore modulated by the electrical field and not by injected carriers. However, as the active region of the device is under a continuous population inversion condition, applying a reverse bias to change the bandgap may cause dramatic photocurrent, depleting the photopumped active region.
U.S. Pat. No. 5,574,738, “MULTI-GIGAHERTZ FREQUENCY-MODULATED VERTICAL-CAVITY SURFACE EMITTING LASER”, issued Nov. 12, 1996, discloses a saturable absorber contained within the VCSEL's distributed Bragg reflector, which may itself be adjusted during fabrication or in operation. Under controllable operating conditions, the saturable absorber, strategically sized and placed, forces the VCSEL to self-pulsate (in the GHz-regime) at rates related to the local intensity, absorption, lifetime, and carrier density of the saturable absorber. In one of the embodiments, efficiency of the saturable absorber may be controlled by the quantum-confined Stark effect. Mode-locked operation, however, is usually very sensitive to the conditions of the device operation and exists only in a relatively narrow range of carefully-optimized conditions.
U.S. Pat. No. 6,396,083, entitled “OPTICAL SEMICONDUCTOR DEVICE WITH RESONANT CAVITY TUNABLE IN WAVELENGTH, APPLICATION TO MODULATION OF LIGHT INTENSITY”, issued May 28, 2002, discloses a device including a resonant cavity. The resonant cavity is delimited by two mirrors and at least one super-lattice that is placed in the cavity and is formed from piezoelectric semiconducting layers. The device also includes means of injecting charge carriers into the super-lattice. One disadvantage of this device is the necessity of using piezoelectric materials. The piezoelectric semiconducting layers are epitaxied on a Cd0.88Zn0.12Te substrate and include a pattern composed of a layer of Cd0.91Mg0.09Te and a layer Cd0.88Zn0.22Te, each 10 nm thick. This pattern is repeated about a hundred times. The device in this patent is a two-terminal device. The separation of carriers in a piezoelectric superlattice causes long depopulation times. Wavelength modulation and intensity modulation are always interconnected in this patent.
An electrooptic modulator based on the quantum confined Stark effect (QCSE) in a VCSEL was disclosed in U.S. Pat. No. 6,611,539, “WAVELENGTH-TUNABLE VERTICAL CAVITY SURFACE EMITTING LASER AND METHOD OF MAKING SAME” issued Aug. 26, 2003, by the inventors of the present invention and herein incorporated by reference. The device includes active media suitable for providing gain and enabling laser action of the device, and a position-dependent electrooptic modulator region. Applying the voltage to the modulator region results in a wavelength shift of the lasing wavelength. The absorption in the modulator region remains small. The device is especially applicable for ultrahigh-speed data transfer using wavelength-modulation.
U.S. Patent Publication 2003/0206741, entitled “INTELLIGENT WAVELENGTH DIVISION MULTIPLEXING SYSTEMS BASED ON ARRAYS OF WAVELENGTH TUNABLE LASERS AND WAVELENGTH TUNABLE RESONANT PHOTODETECTORS”, published Nov. 6, 2003, by the inventors of the present invention and herein incorporated by reference, disclosed high-bit rate data transfer systems based on wavelength-to-intensity modulation conversion. In this approach, a wavelength-tunable VCSEL operates in concert with a wavelength-selective photodetector on the receiver side. Modulation of the VCSEL wavelength transforms into the photodetector current modulation.
There is a need in the art for an ultrafast way to modulate the intensity already at the exit of the device.